This invention relates to a mechanism for orienting a rotary member such as turn table, etc. in a predetermined angular position.
In a machine tool construction it is frequently necessary for accurately positioning a turn table in a predetermined angularly oriented position. This accurate positioning is needed for variety of purposes. For example, when effecting a turning work and a drilling work in a row, the accurate angular positioning of the turn table is required in an automatic operation.
To attain an accurate angular positioning of the turn table, it is desirable to employ a separate servo drive in addition to a main drive. To this end, a separate servo drive is provided solely for effecting the angular positioning of the turn table. An independent drive mechanism is provided for rotating the turn table in a work operation and this independent drive mechanism is rendered inoperative when the positioning movement is being effected by the servo drive.
U.S. Pat. No. 3,483,767 discloses such a separate servo drive mechanism for accurately positioning a rotary spindle; but this prior art device is disadvantageous in that it takes a relatively long time for angularly orienting the rotary spindle in a predetermined angular position.
Japanese Laid-Open Patent Publication No. 53-95379 discloses another prior art which employs two detectors, i.e. a rough detector and a fine detector. This prior art device can shorten the time required for angularly indexing the rotary member, but a control system thereof becomes very complicated.
Co-pending U.S. application Ser. No. 254,268, now U.S. Pat. No. 4,417,486, assigned to same assignee of this application discloses an apparatus for angularly positioning a rotary member such as turn table, etc. by employing two separate motors. First motor is connected to drive the rotary member in a work operation through a power transmission mechanism while second servo motor is connected to drive the rotary member in a servo angular positioning movement through a coupling mechanism including a single position clutch.
Although the apparatus disclosed in this co-pending application is satisfactory in practical use, it nevertheless contains some disadvantages due to employment of the single position clutch. In particular, the single position clutch is expensive and engagement shock load imposed on the single position clutch due to rotary inertia of the rotary member can not be avoided and may sometimes damage the clutch itself.